Chair back



FIG. 1 is a rear perspective view of a chair back showing my new design;

FIG. 2 is a front view thereof;

FIG. 3 is a rear view thereof;

FIG. 4 is a right side view thereof;

FIG. 5 is a top view thereof;

FIG. 6 is a bottom view thereof; and,

FIG. 7 is an enlarged end view thereof, taken along the line 7-7 in FIG. 2. 

CLAIM The ornamental design for a chair back, as shown and described. 